Diesel particulate filters (aka DPFs or PM filters) are gaining acceptance for reducing emissions of harmful particulate matter (PM) from diesel internal combustion engines. DPFs are an aftertreatment technology, generally being placed in the exhaust line downstream of the engine to catch particulate matter within the filter to prevent the harmful material from being released into the atmosphere.
In some circumstances, active regeneration of the DPF is needed to clear the DPF of accumulated particulate matter so that the DPF can continue to operate effectively in removing additional PM from the engine exhaust gas. Regeneration generally occurs by burning off the accumulated particulate matter. Known methods to regenerate a DPF include the use of an electrical resistance heater, and/or the use of an additional injection of fuel, to increase exhaust gas temperatures through the DPF. The high exhaust gas temperature, together with excess oxygen in the exhaust, combine to oxidize the carbon and organic PM in the filter, thereby producing carbon dioxide. A diesel oxidation catalyst (DOC) is also sometimes placed upstream of the filter. As the oxidation reaction in the DOC is exothermic, placement of the DOC upstream of the filter results in increasing the exhaust temperature through the filter, for improved filter regeneration.
Nevertheless, by requiring electrical energy and/or the use of additional fuel, each of these active regeneration methods reduce the overall fuel efficiency of the internal combustion engine system. The energy lost in DPF regeneration can become particularly significant in engine systems with relatively cool exhaust temperatures and/or with particularly high engine-out PM levels, such as low temperature diesel combustion systems that use high levels of exhaust gas recirculation for reduced emissions of NOx.
It is therefore desirable in the art to provide an apparatus and methods for effective DPF regeneration with a reduced fuel efficiency penalty. While some have already advocated one way of improving the efficiency of DPF regeneration, by routing just a small portion of exhaust gas through the DPF during regeneration and bypassing the remainder of the exhaust gas around the DPF (see, e.g., U.S. Pat. No. 4,744,216 to Rao, U.S. Pat. No. 5,063,737 to Lopez-Crevillen, and U.S. Pat. No. 4,276,066 to Bly), such prior systems would result in (i) an increase in emissions during the regeneration process, and/or (ii) potential significant complication of engine management strategies occasioned by significant changes in combustion and exhaust characteristics in each of the engine's cylinders during the regeneration event.